Small misalignments between the Eigenmode of the cavity and the input beam can be expressed as the latter having a small amount of power in the Hermite-Gauss \(U_{01}\)  and \(U_{10}\)  modes defined by the cavities Eigenmode basis. Equation REF  shows how this can be quantified in terms of the parameter \(\epsilon \)  by writing the input laser as a superposition of the spatial eigenmodes of the cavity [1]}.
\(U_{\rm in} \approx A[U_{00}+\epsilon _x U_{01}+\epsilon _y U_{10}]\) 
